Computer technology is expanding at an enormous rate with seemingly boundless applications in virtually all fields of human endeavor. While initially confined to the domain of experts knowledgeable about and familiar with the hardware and software design capabilities of a given system, this technology has been extended to an extremely broad range of individuals of widely varying skills. At the forefront of this evolution is the desirability of providing simple but efficient means for operator interaction with what otherwise may clearly be a highly sophisticated computer system. To this end, great strides have been made to simplify the operational characteristics of computer systems in order to render the same accessible to even those who have no knowledge or specific educational background enabling them to understand the intricacies of a system with which they must deal. Along these lines, a plethora of devices are now provided for an operator to interact with a host system by direct contact with a visual display, thereby eliminating the need to resort to the use of a keyboard or the like.
Such direct interactive devices admit of many advantages over inputing data via, e.g., a keyboard. Many individuals are timid in "communicating" with a computer and this apprehension oftentimes contributes to error where information is introduced through a keyboard. The problem is exacerbated since the common tendency is to focus on the keys rather than on the visual display associated with the keyboard with consequent typographical errors; in turn reinforcing the psychological problem of interaction by the operator. Even single-function keys have not entirely overcome these problems and, in further point of fact, there is a practical limit to the ability of a manufacturer to provide a keyboard with a range of single-function keys coextensive with the contemplated uses of the system. Accordingly, the art has responded with various suggestions for visually interactive means permitting the operator to interface with a computer directly through a visual display therefor. Among these presently recognized in the art may be mentioned a light pen, a light beam array, capacitance-type switches, piezoelectric switches, surface accoustic wave transducers, resistive contacts, and resistive analog approaches.
Historically, one of the first commercial devices for direct operator interaction with a computer terminal through a visual display was implemented in the form of a light pen. In the main, this class of device is represented by a photodetector housed within an elongate member in the shape of a conventional pen. The light pen is manipulated by placing the photocell in proximate contact with the face of a CRT display at a desired location. As the raster scans the face of the CRT it causes illumination sufficient to trigger the photocell in the pen, the physical location thereof being correlated to the raster position at the time a pulse is detected. Usually a switch is included on the pen to enable the device when it is put in use; in order to avoid spurious signals. Two important points are pertinent as respects this light pen device. First, the vast majority of CRT driver (controller) chips have routinely included a light pen input to accommodate the device either as an integral component of a system or as an optional accoutrement. To date, the inclusion of a register accessible for a light pen input remains notwithstanding the second point worthy of comment--the light pen has not been a very successful approach from a commercial point of view. Its acceptability has principally been limited to the inconvenience of either holding the device in order to have it readily available or repetitively grasping it for use with the system. Accordingly, although technologically very acceptable, the device has not proven to be desirable from a user's standpoint.
Light beam arrays interrupted by the presence of a finger of the operator or other intrusive object have been proposed as a means for operator interaction with a host system. In some instances, these devices are implemented as horizontal and vertical arrays of light emitting diodes or the like and associated detectors disposed about the periphery of, e.g., a CRT display. The interposition of either an operator's finger or another opaque object interrupts a given light beam, the detection of which allows the characterization of the location of the interposed object. In one variant the light beams are columnated in order to provide physical discrimination of points within an X-Y gridwork. In other variations, the LED's are cycled "on" and "off" in a specific sequence so that at any given moment only appropriate ones of the emitter/detector pairs are energized in order to avoid the cost of beam columnation while nonetheless providing spatial discrimination. Still other approaches within this realm of optical touch panels include the use of a continuous light source with charge-couple devices ("CCD") configured as linear image sensors. These types of panels require a continuous light source about three sides of the display with two CCD linear image detectors mounted in the corners connecting the remaining side. The CCD's are mounted at angles whereby shadows caused through interposition of an object within the light field over the display will be received by both. A focusing device screens shadows from the CCD's except those from which precise touch locations may be determined by appropriately referenced trigonometric equations. Regardless of the specific mode of implementation, each of these approaches suffers one or more significant disadvantages. Each is (to varying degrees) particularly susceptible to intrusion from ambient lighting, and particularly the continuously illuminated panel last-mentioned above. Columnating a matrixed array of LED's is a fairly costly approach. While that is minimized to a certain extent by sequencing the emitter/detector pairs, the curvature of a conventional CRT display poses some significant problems in initial alignment and subsequent detection irrespective of the format of emitters and detectors. Then too, such an array may oftentimes require considerable hard-wiring to provide input/output function for the components constituting that array.
Capacitive touch panels are also known, and typically rely upon a glass panel with conductive surfaces screened on both front and back. In this way the glass intermediate the conductive surfaces serves as the dielectric. The touch-sensitive capacitance switch usually consists of two such capacitors in series which have identical capacitance values correlated to an engineering design for the average capacitance of human touch at about 50 picofarads. In operation, a low-level signal is continuously pulsed through this switch array; which signal is shunted to ground through the body capacitance when a finger touches the top plate of the switch. Spatial discrimination can be achieved by taking into account the timing of the pulse signal. Another, more elaborate approach involves the continuous monitoring of the reference capacitance of the untouched panel by a microprocessor. A touch by a finger tip causes a change which is identified and interpreted by associated electronics. While these capacitive touch panels extend the environmental range of adaptability for touch-sensitive overlays, they nonetheless suffer their own indigenous problems. Usually, the same must be designed with fairly wide spacing between signal traces in order to minimize the effects of stray capacitance. This can place some rather severe constraints on the number of touch-sensitive areas per overlay. This contributes, in turn, to the problem of limiting the field (i.e., physical presentation) of information on the display; in essence, the overlay defines the display field--a most undesirable result. Those overlays incorporating a microprocessor-based tracking system to improve reliability carry with them a proportionately higher cost.
Piezoelectric switches rely upon that (i.e., piezoelectric) effect to sense the presence of a touch. Somewhat related to the piezoelectric switch is the use of a surface accoustic wave to provide another type of touch-sensitive overlay. Instead of a pair of conductive surfaces screened on both sides of a glass substrate as was the case with respect to the capacitance switch mentioned above, a series of piezoelectric transducers are mounted along one X and one Y axis of the glass plate. These transducers generate an accoustic wave of ultrasonic (e.g., 4 MHz) frequency on the surface of the glass (usually about 1/16"). Touching the surface causes a reflection of the transmitted wave or echo which is recognized by other transducers. The time between the outgoing transmitted waves in both the X and Y direction and their reflections may be measured and are characteristic of the distance of the touched point from each of the X and Y axes. These piezoelectric overlays are rather difficult and hence expensive to manufacture. They are also very susceptible to environmental problems; the surfaces must be maintained scrupulously clean and free from scratches or nicks lest erroneous or spurious results obtain.
Membrane-type switches have also been proposed for operator interaction with a computer system. One approach is embodied in a resistive contact switch wherein the switch matrix is etched or otherwise imposed on an optically-clear polymeric substrate. Two such substrates are disposed in face-to-face relationship separated one from the other by a slight air gap. An operator pressing the outer one of the substrates will cause both pieces to touch and form a contact area which can be detected by digital circuitry for purposes of spatial discrimination. Another type of overlay within this ambit employs an A/D converter for both the X and Y axes. Comparison of the resistance values allows for spatial discrimination.
The foregoing generally constitute the types of overlays currently or heretofore in use. Those of simpler design have been found to suffer certain disadvantages. In addition to the drawbacks enumerated in general above, parallax problems can be significant. Alignment problems also pose considerable difficulties. For example, where an array of either switch contacts or light means are fixedly disposed over or about the periphery of a CRT display, the display image itself may vary in position over the course of time and/or in response to temperature fluctuations whereas the fixed overlay obviously will not. Hence, where a certain response is dictated by the location at which the display is touched, a shifting of the character locations (e.g., due to vertical or horizontal misalignment of the CRT) will yield an erroneous result. Interface circuitry is oftentimes unwieldy due, e.g., to numerous wires required by gridwork arrays of contact points. Efforts to overcome these drawbacks have resulted in devices which are quite complex and hence costly; eliminating adaptability for many applications if for no other reason than economic disincentive. Furthermore, many of the overlays heretofore in use irrespective of complexity or cost considerations suffer the drawback of limiting the useful presentation of information on the associated display; the overlay itself effectively defining the limits of the visual display.
Accordingly, the need exists to provide an improved touch-sensitive overlay for operator interaction with a computer system; which overlay is simple in construction, of low-cost, but which is reliable in operation regardless of any variations in the absolute position of the character locations on the display device.